The invention relates to substantially hermetically sealed nickel-cadmium cells which work on the oxygen cycle principle, and which have a charge and discharge reserve at the negative electrode and have a diffusion body at the side of the negative electrode facing away from the positive electrode.
Substantially gas tight nickel-cadmium cells usually work on the oxygen cycle principle, i.e., the positive capacity oi so designed that the positive electrode is completely charged. This avoids a generation of hydrogen at the negative electrode. The oxygen is reduced at the negative electrode so that, even with continued charging, the negative is never fully charged and there is, therefore, no risk of hydrogen generation. The oxygen is, however, usually slowly reacted so that, depending on charge current level and overcharging state, an oxygen partial pressure of a few bar prevails towards the end of charging. Since the cell is filled with air during construction, the interior space contains the non-reactible constituents of air, in addition to the quantity of oxygen which varies considerably in periodic cycles. In particular, some non-reactible constituents include nitrogen and small quantities of hydrogen, as well as water vapor from the electrolyte.
Although it is known that after a certain service life or after complete discharge (Varta Fachbuchreihe, Vol. 9, "Gas Tight Nickel-Cadmium Accumulators", 1978, p. 74), or under certain conditions with charging lasting for days (German Published Examined Patent Application No. 1,127,418), the oxygen in the cell may be reacted to such an extent that a slight underpressure prevails in the cell. Thus, the use of an over-pressure-resistant housing for such cells has hitherto been unavoidable. The cells therefore have a metallic housing in cylindrical or prismatic form which is capable of withstanding the internal pressure without substantial deformation.
However, German Patent Application No. 2,742,869 shows a substantially gas tight nickel-cadmium cell which has a prismatic, shallow plastic housing. The bulging of the housing as a result of the overpressure occurring towards the end of charging is counteracted by means of a continuous central housing plug which holds the base and lids together.
It is furthermore known that, as a result of the incorporation of special gas diffusion bodies which are arranged on the side of the negative electrode facing away from the positive electrode, the oxygen produced during charging or overcharging can be fed systematically to the negative electrode and reacts there particularly rapidly (for example, German Unexamined Patent Application No. 2,907,262). However, in this arrangement, the rate of reaction of oxygen (oxygen consumption rate) at the negative electrode is still so slow that an overpressure-resistant housing continues to be necessary.
An object of the present invention is to provide a nickel-cadmium cell which works on the oxygen cycle principle as discussed above in which the oxygen consumption rate is so high that overpressure is no longer formed in the cell, and the latter can be constructed with a particularly light housing.
These objects and other objects are achieved by providing a hermetically sealed nickel-cadmium cell aparatus which works on the oxygen cycle principle and has a charge and discharge reserve at the negative electrode. Any gases which cannot be reacted in the cell have a partial pressure below 0.4 bar.
Surprisingly, the present invention has shown that an overpressure no longer builds up in the cell, if the partial pressure of the gases which cannot be reacted in the cell is below 0.4 bar. The removal of the non-reactible gases, in particular, nitrogen, from the gas space of the cell, results in a very considerable increase in the oxygen consumption rate. As a result, oxygen pressure is reduced by a factor of 3 to 4 compared with a conventional cell for the same charge current level and the same charge condition. This applies even at high charging rates of up to 2 C.sub.N A (current in A of double the nominal capacity in Ah).
The lower the partial pressure is of the gases which cannot be reacted in the cell, the better is the oxygen consumption rate. As soon as the partial pressure of the gases which cannot be reacted in the cell rises above 0.4 bar, the behavior of the cell increasingly approximates a conventional cell. According to advantageous features of a particularly preferred embodiment, the partial pressure of the gases which cannot be reacted in the cell is below 0.1 bar.
These non-reactive gases can be removed before the final sealing of the cell. According to certain preferred embodiments, these gases are removed by thoroughly flushing with oxygen, for example. However, in particularly preferred embodiments, the removal of these non-reactive gases is achieved by evacuation of the cell by means of a vacuum pump. It was discovered that if partial pressures of less than 0.4 bar are maintained, an overpressure does not occur in any operating state of the cell, even with high charging currents. Such cells can therefore be given plastic housings, preferably in the prismatic or shallow construction, without any particular further pressure-resistant measures. The external atmospheric pressure forces the assembly of plates together via the plastic housing and ensures a good hydraulic contact. The essential technical advantages of such cells are: the lack of risk of explosions with serious consequences in the event of incorrect use or incorrect operation in mines or in manned space travel, a saving in weight compared with metal housings, a very simple manufacture of the housing by injection molding processes and a simple insulation of the terminal lead throughs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.